1. Field of the Invention
The present invention relates to a liquid crystal display and a method of driving the same and, more particularly, to a liquid crystal display which is driven by an inversion driving method.
2. Description of the Related Art
Generally, a liquid crystal display structure includes a liquid crystal layer having a dielectric anisotropy sandwiched between two opposing substrates. An electric field is applied to the liquid crystal layer with various in strength, thereby controlling light transmission and displaying the desired picture image.
A plurality of pixel electrodes are arranged on one of the substrates in a matrix form, and counter electrodes are arranged on the other substrate such that they correspond to the pixel electrodes. Each of the electrode pair operates with the interposed liquid crystal thereby forming a liquid crystal cell, and the light transmission characteristic of the liquid crystal cell is selectively controlled by applying voltage to the electrode pair, thereby displaying the desired picture image.
The above-structured liquid crystal displays are representative of portable flat panel displays. Among them, thin film transistor liquid crystal displays (TFT LCD) with thin film transistors as switching circuits have been extensively used.
In such a thin film transistor liquid crystal display, thin film transistors are formed on a substrate such that they correspond to pixels arranged in a matrix form. The substrate with the thin film transistors formed thereon is usually called the “thin film transistor array substrate.” A pixel electrode is formed at each pixel of the thin film transistor array substrate such that it receives picture signals depending upon the control of the corresponding thin film transistor. Gate and data lines are formed on the thin film transistor array substrate such that they are connected to the pixel electrodes via the thin film transistors. The data lines cross over the gate lines to thereby define pixels in a matrix form. The gate lines are connected to output terminals of gate driving integrated circuits to receive gate signals and transmit them to the pixel electrodes. The data lines are connected to output terminals of data driving integrated circuits to receive picture signals and transmit them to the pixel electrodes.
FIG. 1 illustrates the conceptual structure of a conventional liquid crystal display. In the drawing, G1 to Gm indicate the gate lines, S1 to Sn indicate the data lines, P indicates the pixel electrode, and TFT indicates the thin film transistor.
When the driving voltage of the same polarity is continuously applied to the liquid crystal cell, the pixel electrode and the counter electrode change electrochemically due to the saturation of ionic impurities in the liquid crystal material, and this deteriorates the display sensitivity and the brightness.
In order to prevent such a defect, the polarity of voltage applied to the liquid crystal cell is required to be inverted in a cyclic manner, and this driving technique is called the “inversion driving technique”. Such inversion driving techniques include a frame inversion where the polarity is inverted per a frame, a line inversion where the polarity is inverted per a line, and a dot inversion where the polarity is inverted per a pixel. Among the techniques, the line inversion or the dot inversion is mainly used.
The dot inversion driving technique applies the driving voltages of the opposite polarity to the two pixel electrodes neighboring each other in the column and row directions. A driving voltage of positive polarity is applied to one of the neighboring pixel electrodes, and a driving voltage of negative polarity is applied to the other pixel electrode. This polarity state is inverted per each frame.
The dot inversion driving techniques has two methods. One is a 1 dot inversion driving where the vertically and horizontally neighboring pixel electrodes bear opposite polarity. The other is a 2-1 dot inversion driving where the horizontally neighboring pixel electrodes bear the opposite polarity but the polarity of the vertically neighboring pixel electrodes is inverted per two rows. The 2-1 dot inversion driving technique has several advantages over the 1 dot inversion driving technique. Reduced power consumption and no-flickering at the window screen are examples.
FIG. 2A illustrates the polarity state of pixels in a liquid crystal display where the 2-1 dot inversion driving technique is used. FIG. 2B illustrates the brightness of the pixels shown in FIG. 2A. FIG. 2C illustrates the voltage storage of the pixels shown in FIG. 2A.
In the 2-1 dot inversion driving technique, voltages of the same polarity are applied to the pixel electrodes per two pixel rows. For this reason, as shown in FIG. 2B, the voltage storage between the vertically neighboring pixel electrodes between up and down varies to deteriorate brightness over the entire screen area and forms dim horizontal lines.
As shown in FIG. 2B, when the first pixel row #1 and the second pixel row #2 are charged with the positive (+) polarity, and the positive (+) data is inverted into the negative (−) data at the third pixel row #3, an AC current is generated due to the parasitic capacitance between the pixel electrodes at the second pixel row #2 and the pixel electrodes at the third pixel row #3. This deteriorates the charge rate of the pixel electrodes at the second pixel row #2.
Therefore, among the two pixel rows that receive gray scale voltages of the same polarity, the brightness at the second pixel row becomes lower due to the charge rate deterioration compared to the first pixel row, thereby generating faint difference in brightness per a pixel row, that is, per a gate line.
Furthermore, when voltage delay occurs due to the slew rate without applying an ideal square wave, the charge rate deteriorates at the first pixel row. As a result, in the two pixel rows under receiving voltages of the same polarity, the brightness at the first pixel row is reduced compared to the second pixel row. Therefore, brightness difference occurs even at the pixel rows receiving voltages of the same polarity. Consequently, horizontally extended bands are displayed at the screen while deteriorating the display characteristic.